Process for electroplating metals into microscopic recessed features

ABSTRACT

Several techniques are described for reducing or mitigating the formation of seams and/or voids in electroplating the interior regions of microscopic recessed features. Cathodic polarization is used to mitigate the deleterious effects of introducing a substrate plated with a seed layer into an electroplating solution. Also described are diffusion-controlled electroplating techniques to provide for bottom-up filling of trenches and vias, avoiding thereby sidewalls growing together to create seams/voids. A preliminary plating step is also described that plates a thin film of conductor on the interior surfaces of features leading to adequate electrical conductivity to the feature bottom, facilitating bottom-up filling.

REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. Provisional PatentApplication 60/105,699, filed Oct. 26, 1998, which is incorporatedherein by reference.

BACKGROUND OF THE INVENTION

1. Technical Field

This invention relates to electroplating and, in particular, toelectrochemical deposition of metals, most particularly to theelectrochemical deposition of copper into microscopic recessed featuressuch as high aspect ratio trenches and vias as may occur in thefabrication and packaging of integrated circuits.

2. Description of Related Art

The art of integrated circuits is moving irresistibly towards increaseddensity of components and faster operating speeds. One problemencountered in decreasing.-the size of components fabricated on anintegrated circuit relates to the conductivity of metallicinterconnections. Conventional integrated circuits use aluminum as aconductor but for future generations of submicron components, theconductivity of aluminum is not sufficiently high to give desiredperformance. Smaller dimensions for conductive interconnections lead tohigher resistance and degraded circuit performance. The trend in modemintegrated circuit design is to substitute higher conductivity copperfor aluminum conductors.

While copper apparently has sufficiently high conductivity to handle theforeseen submicron electronic components, its use brings challenges aswell. For example, copper (Cu) tends to diffuse readily into theinsulator and other layers making up the integrated circuit,necessitating the interposition of special barrier layers to prevent Cudiffusion. Tantalum (Ta) and/or tantalum nitride (TaN) are common copperbarrier layers. Such barrier layers may play dual roles; preventingunwanted diffusion of Cu and providing improved adhesion between the Cumetal and the underlying layer.

Conductive interconnections on integrated circuits typically take theform of trenches and vias. In modem submicron integrated circuits,trenches and vias are typically formed by a “damascene” or “dualdamascene” process as described, for example, in the reference ULSITechnology, Eds. C. Y. Chang and S. M. Sze (McGraw-Hill, 1996, pp.444-445.) In damascene processing, an interlayer dielectric (typicallySiO₂) is deposited atop a planarized layer containing, for example, ametal via. The top dielectric layer is patterned and etched, typicallyusing conventional photolithographic procedures. Metal is then depositedinto features and on the flat field region atop the features, typicallyfirst by CVD, PVD and then by electrodeposition. The metal layer istypically planarized resulting in the desired metallic pattern. Dualdamascene processing is similar but makes use of two patterning andetching steps and typically fills features with metal spanning more thanone layer in a single metallization step. A more complete description ofdamascene and dual damascene processing is found in the cited reference.

Thus, as the art moves towards integrated circuits having reducedfeature sizes, it becomes increasingly difficult to form electricallyconductive metallizations such as vias, contacts and conductors.Techniques for forming such metallizations include physical vapordeposition (PVD), chemical vapor deposition (CVD), plasma enhancedchemical vapor deposition (PECVD) and electrochemical deposition (alsoreferred to as electroplating or electrodeposition) of metals such ascopper. Electroplating is particularly well suited for the formation ofsmall embedded damascene feature metallization due to the ability toreadily control the growth of the electroplated film for bottom-upfilling without voids, and due to the superior electrical conductivitycharacteristics of the electroplated film. However, there are alsoseveral obstacles which need to be overcome to fully realize theseadvantages.

One challenge facing dual damascene processing techniques noted above isthe difficulty of initiating the growth of the metal film withinrecessed features without forming voids or seams. In typical PVD andsome CVD processes, metal may preferentially deposit near the top ofrecessed features leading to a “bottleneck” shape. Further plating ofmetal onto the bottleneck may result in sealing the top of the featurebefore completely filling the feature with metal, creating thereby avoid. Voids increase the resistance of the conductor over its designedvalue absence of planned-for conductor. Also, trapped electrolyte insealed voids may corrode the copper. This may lead to degraded deviceperformance or device failure in extreme cases. It would be desirable toprovide electroplating processes that reduce or avoid such problems.

Another factor in metallization that needs to be considered is the timethe metallization process takes to complete. As integrated circuitsincrease in complexity, the number of processing steps typically alsoincreases. It is important in process economics (that is, keeping thecost per unit low) that such processes are reasonably rapid. Slowprocesses are disadvantageous in that increasing the work-in-progressincreases the capital needs of the production process. A relatedconsideration is the cost of the equipment needed to perform therequired plating process. Thus, achieving reasonably high processingspeeds and the ability to use less costly equipment are among the goalsto be sought in an electroplating process.

A field layer of copper typically has a thickness from approximately1000 Å to approximately 3000 Å lying atop a barrier layer of typicallytungsten (W) Ta, TaN among other choices. Ta/TaN are the typicallypreferred materials for the barrier layer due to their relativelysuperior ability to mitigate the diffusion of Cu. Diffusion of Cu intothe dielectric layer may ruin the circuit. An additional function of thebarrier layer is to provide improved adhesion between the Cu field layerand the underlying dielectric layer. These barrier layers typically havethicknesses from approximately 100 Å to approximately 400 Å.

FIG. 1 is a top plan view of a typical section, 12, of a substrate, 10.Substrate 10 can be any article to be electroplated such as asemiconductor wafer upon which integrated circuits are fabricated.Formed in (or in a layer lying over) substrate 10 may be one or moretrenches (a typical example is denoted by 14 ) and vias 16, 16 a, 18.Although only a single trench 14, and three vias 16, 16 a and 18, areillustrated for simplicity of discussion, it is understood thattypically a large number of trenches and vias may be formed in substrate10. It is common in a typical damascene or dual-damascene structureoccurring in the fabrication of integrated circuits that vias occur intrenches to provide electrical contact between a conducting trench and adifferent level. This is depicted as 16 a in FIG. 1. However, it is moreconvenient to describe trenches and vias in a non-overlappingconfiguration as depicted by 14, 16 and 18 for purposes of illustration,understanding thereby that vias occurring in trenches, 16 a, areincluded.

Trench 14, vias 16, 16 a and 18 are typically filled with anelectrically conductive material such as a metal, e.g. copper, to formelectrically conductive metallizations. For example, trench 14 may befilled to form a single layer conductor. As further examples, vias 16,16 a and 18 may be filled to form electrically conductive vias betweeninterlevel conductors or to form contacts with semiconductor regions.

While a typical Cu seed onto which Cu is electroplated layer is 1000 Åto 3000 Å thick, the feature widths to be plated are commensurate insize. Present features are around 3000-4000 Å (0.3-0.4 micron, μm) andfuture features are expected to be in the range 1000-2000 Å. Thus, thenumber and size of the features can have a significant fractional effecton the projected surface area to be bottom-up fill electroplated and,therefore, on the current that must be delivered to effectelectroplating in a reasonable time.

FIG. 13 depicts the effect of increasing the number of features on thecurrent required to fill the trenches (for a fixed fill time). Thismeans that the current required to fill features in a specified amountof time increases with the fraction of features on the surface. Inaddition to process economics, we demonstrate herein that the process ofthe present invention make use of the time for electrochemical reactionsto occur on surfaces in effecting proper metal filling of features.Thus, the number of features on the surface to be coated (featuredensity) as well as the aspect ratio of the features determine thecurrent needed to effect plating in a specified time.

FIG. 14 depicts the increase in surface area as an increasing fractionof the surface contains features, also illustrating the effect ofincreasing aspect ratio “AR,” for either trench or via. FIG. 14 presumesall trenches or all vias are the same size for purposes of illustrationwhile practical integrated circuits will have both. This reinforces theconclusion of FIG. 13 that current may vary substantially from case tocase (wafer type to wafer type) depending on the density andcharacteristics of the features.

FIG. 2 is a cross-sectional view of substrate 10 along the line II-II ofFIG. 1. Referring to FIGS. 1 and 2, a seed layer 20 is typically formedover the entire substrate 10. Seed layer 20 is typically a conductivelayer overlaying a barrier. The conductive layer is typically asputtered (PVD) copper film although other conductors and other methodsof deposition (such as CVD, PECVD, etc.) are not excluded. The barrierlayers are typically Ta, TaN, Ti, TiN. The function of the seed layer isto allow electrical current to be distributed across substrate 10thereby facilitating electroplating. Seed layer 20 typically covers theflat principal surface 22 of substrate 10 (hereinafter “field region22”) and also lines the inside of trench, 14, and vias 16, 16 a and 18.

In conventional electroplating, the thickness of seed layers 20 onsidewalls 14S, 16S, 18S of trench 14, vias 16, 18, respectively, iscommonly significantly less than the thickness on field region 22. Thiscan be understood by considering a simple geometric model wherein seedlayers 20 are assumed to be deposited uniformly on field region 22 andthe material which would have been deposited on field region 22 overtrench 14, via 16 or 18 is evenly distributed within the respectivetrench or via. Thinning of the deposited layers in trenches and viasoccurs according to this model due to the geometric fact that the trenchor via presents more surface area to be coated by a given amount ofcoating material than would a substantially flat field region. It isuseful to consider such layer thinning in terms of the aspect ratio(“AR”) of the feature. The AR of a trench or via is defined as the ratioof depth to width of the feature. Vias are depicted as square in FIG. 1for convenience only. Vias can be square, circular or other crosssection. The discussion of vias herein illustrated by reference tosquare vias is readily modified to apply to circular vias or to viashaving other shapes as well. The AR is thus taken to be the ratio of thevia's depth to a typical linear dimension in the plane of field region22. A more precise definition of AR is not necessary for a descriptionof the present invention.

Sidewall thinning according to this model is illustrated in FIG. 3.which depicts on the vertical axis the ratio of the feature sidewallthickness (trench or via) to the field region thickness of seed layer 20depicted as a function of the AR of the feature. As shown in FIG. 3, asthe AR becomes large, the thickness of seed layer 20 on sidewalls 14S,16S, 18S can decline to less than 10% of the thickness of seed layer 20on the field region 22.

In practice, the difficulty of depositing metal uniformly on the sidesof trenches and vias to form a seed layer is further exacerbated by thefollowing effects: 1) The generally directional nature of the PVDsputtering process, 2) greater deposition at the opening of the feature(“necking”) and 3) The relative inaccessibility of the feature tospecies transported by diffusion. Thus, in practice the portions of thesidewalls near the bottom of the feature tend to receive the thinnestseed layer.

Referring again to FIG. 2, the relative thinness of seed layer 20 onsidewalls 16S (for example) results in a relatively high electricalresistance R between field region 22 and the bottom 16B of via 16. Thisresistance R impedes electroplating current distribution to bottom 16Band consequently inhibits or prevents formation of an electroplatedlayer on bottom 16B. For similar reasons, increased resistance resultsbetween seed layer 20 and 14B, 18B, inhibiting or preventing formationof an electroplated layer on the bottom 14B of trench 14, and 18B of via18, respectively. Similarly, increased resistance results between seedlayer 20 and 14B, 18B.

FIG. 4 is a cross-sectional view of substrate 10 along the line II-II ofFIG. 1 after electroplating in accordance with conventionalelectroplating methods. As shown in FIG. 4, an electroplated layer 24has been electroplated on substrate 10 and in trench 14, vias 16, 18.Often due to the increased resistance encountered in delivering currentto bottom 16B of via 16, a teardrop shaped void 26 can be formed in via16 by such electroplating methods. The specific shape of the voids orseams formed in a feature generally depends on the AR of the feature,the electroplating conditions and other factors.

Since via 18 has a smaller AR than via 16 (in this illustration), seedlayer 20 may have a thickness on sidewalls 18S sufficient to support anelectroplating current to bottom 18B. Nevertheless, seams/voids mayform, depicted as 28, if conditions for bottom-up filling (current,electrolyte, additive concentration, etc.) are not achieved

Voids and seams in electroplated features are undesirable for severalreasons. Voids and seams disrupt the electrical continuity of theresulting metallization and hence unpredictably increase the electricalresistance. In extreme cases, voids and/or seams may result in an opencircuit. Furthermore, the voids and seams can trap impurities, includingthe acid plating solution. Entrapped acid plating solution can corrodethe circuit and add significant internal pressure as the circuit/chipheat during operation. Device failure may result.

Accordingly, the present invention relates to an improved method ofreliably electroplating high AR features including methods of reducingor eliminating the formation of voids and seams in electroplatedfeatures.

BRIEF SUMMARY OF THE INVENTION

Electroplating metals onto semiconductor surfaces is one common way forproviding metal contacts and conductors in the manufacture of integratedcircuits. However, as integrated circuit features become ever smaller,electroplating without the formation of seams and/or voids becomes morechallenging. The present invention identifies several sources of suchseams/voids within the electroplating process and describes severalmitigating techniques.

Deposit of a conducting seed layer onto a substrate and microscopicrecessed features often does not provide adequate metallization of thefeature walls, leading to poor electroplating. The poor electricalquality of feature sidewalls and bottom is observed to worsen when thesubstrate is brought into contact with the electroplating solution. Thepresent invention describes techniques of cathodic polarization to avoidthe exacerbating effects of contact with electroplating solutions.

Avoidance of seams/voids requires bottom-up filling of recessedfeatures. The present invention describes diffusion-controlledelectroplating in which additives that suppress electroplating arepreferentially removed from the feature bottom, leading to preferentialelectroplating of the bottom and, hence, bottom-up filling.

Bottom-up filling requires adequate electrical conductivity from thefield region of the substrate to the bottom of the feature. Apreliminary plating step is described in the present invention leadingto thin films of conductor from the field region to the feature bottom.These films are deposited by an electrodeposition process making use ofelectroplating-suppressing additives to insure uniform film formation.

As bottom-up filling proceeds, high aspect ratio features become lowaspect ratio features, amenable to conformal electroplating techniquesto finish the filling of trenches and vias. Techniques for filling verylow AR features (“pads”) in reasonable short processing times are alsodescribed.

BRIEF DESCRIPTION OF THE DRAWINGS

All drawings are schematic and not to scale.

FIG. 1 is a top plan view of a section of a typical substrate to beelectroplated.

FIG. 2 is a cross-sectional view of the substrate along the line II-IIof FIG. 1.

FIG. 3 is a graph of the ratio of feature sidewall thickness to thefield region thickness of a seed layer versus the respective feature AR.

FIG. 4 is a cross-sectional view of the substrate along the line II-IIof FIG. 1 after electroplating.

FIG. 5A is an enlarged view of region 100 of FIG. 2 illustrating earlystage degradation of seed layer.

FIG. 5B is an enlarged view of region 100 of FIG. 2 illustratingcorrosion of copper layer and oxidation of tantalum layer likely tooccur in conventional electroplating.

FIG. 6 is an enlarged view of region 100 of FIG. 2 following theinitiation phase of the present invention.

FIG. 7 is a cross-sectional view of substrate 10 along the line II-II ofFIG. 1 after the initiation phase of the present invention.

FIG. 8 is the cross-sectional view of FIG. 7 during the early stages ofthe bottom-up filling phase of the present invention.

FIG. 9 is the cross-sectional view of FIG. 8 at a later time in thebottom-up filling phase of the present invention than the depiction ofFIG. 8.

FIG. 10 is the cross-sectional view of FIG. 9 at a later time in thebottom-up filling phase of the present invention than the depiction ofFIG. 9.

FIG. 11 depicts a typical cathodic-anodic-cathodic current duty cyclefor the bottom-up filling phase according to the present invention.

FIG. 12 is an enlarged view of FIG. 9 depicting redistribution ofmaterial as facilitated by the reversal of electroplating current.

FIG. 13 depicts the increased current required for bottom-up filling asthe percentage of wafer surface occupied by trenches increases forspecified process times.

FIG. 14 depicts the % surface area increase as a flnction of the %features for various feature aspect ratios (“AR”).

FIG. 15 demonstrates that for typical additive concentrations present inelectroplating baths, the initial additive present in trenches and viasis insufficient to coat the feature surfaces with a significant fractionof a monolayer, leading to an initial period of effectivelyadditive-free plating behavior in features.

FIG. 16 shows the estimated induction period for plating additiveadsorption inside features.

FIG. 17 shows the estimated induction period for adsorption of platingadditives.

FIG. 18 illustrates the thermodynamic basis for the formation of Ta₂O₅on feature sidewalls.

FIG. 19 shows the equivalent circuit model for trench and via filling.

FIG. 20 illustrates model calculations for the effective electricalresistance and voltage drops in the filling of small features.

FIG. 21 is a graphical depiction of nucleation phenomena in features.

FIG. 22 is an example of a possible bipolar pulse plating waveformappropriate for the filling phase (phase 3), depicting cathodic(growth/deposition) current, off time, anodic time during which copperis removed (re-dissolved), potentially another off time and resumptionof the cathodic current. To insure net plating of copper, thetime-current relationship must obey the constraint that thetime-integrated cathodic current exceed the time-integrated anodiccurrent. This bipolar pulse plating waveform may be particularly usefulin preventing the formation of bottlenecks as the anodic pulse canremove developing bottlenecks. Typical anodic currents are longer thanabout 5-10 milliseconds in duration.

FIG. 23 shows the results of bipolar pulse plating waveform for fillingphase depicting improved filling with anodic pulse (right) in comparisonwith dc process (left).

FIG. 24 is a summary of optimization for bipolar pulse plating forfilling phase derived from series of experimental results for varioustypes of pulse plating waveform. Generally, higher anodic currentsimprove top filling (avoidance of voids near the top of features). Lowercathodic currents tended to improve filling.

FIGS. 25-32 are examples of bipolar pulse plating waveforms for thefilling phase 3.

FIG. 33 is a summary of conclusions derived from tests depicted in FIGS.25-32.

FIG. 34 is a comparison of Hollow Cathode Magnetron (“HCM”) PVDdeposition with Ionized Magnetized Plasma (“IMP”) showing more severelydiscretized films on the feature walls resulting from the IMP process aswell as more pronounced necking effects.

FIGS. 35 and 36 show experimental current sweep for various sweep andcurrent conditions for phase 3 bottom-up filling. The resultsdemonstrate current sweeping is effective in bottom-up filling of allaspect ratio features as well as bottom-up filling repair.

FIG. 37 gives examples of plating during initiation phase, phase 2, atvarious currents and for various times as shown. The features are viasof approximately 1 μm depth and width as noted in Field 3 and Field 4.The structure of the film is shown after initiation plating according tothe specified conditions. 0.5 amp shows significantly more metal at thebottom of the via than 0.1 amp.

FIG. 38 depicts corrosion upon phase 1 (entry) of the wafer into theelectroplating bath in which a time delay (“induction time”) occursbefore the cathodic polarization is introduced, as would be required topermit complete wetting of the wafer, for example. An induction periodof as short as 2 seconds shows depletion of copper.

FIGS. 39 and 40 show the effect of conformally depositing another layer(“backfilled”), amorphous silicon in these figures to increase theaspect ratio. This is a technique to study the electroplating of smallerfeatures when such features may not be reliably fabricated by presenttechnology. Different magnifications are shown.

FIG. 41 shows the initiation phase plating (phase 2) using low currentssuch that the relative resistance of feature top and bottom is high incomparison with diffusion resistance. At low current, the relativeresistance of surface reaction to diffusion is high. Thus, conformalplating occurs without substantial depletion effect (depletion of copperdue to reaction occurring faster than diffusion to the reaction site).Slow plating rate favors conformal plating. Additionally, low platingcurrent has a small additive depletion effect, further increasingconformality. Bottom-up filling does not occur at these low currentlevels, 0.5 amp.

FIG. 42 relates to the initiation phase of FIG. 41 and variousconditions tested.

FIG. 43 relates to unipolar pulse conditions for the initiation phase,phase 2.

FIGS. 44-46 depict the relative merits of unipolar pulsing or the lackthereof in the initiation phase, phase 2.

FIG. 47 relates to phase 1 entry phase and an induction period of 2seconds following the entry of the wafer into the plating bath before acurrent of 7 amps is turned on. Poor film nucleation is observed.

FIG. 48 has the conditions of FIG. 47 but the initial 7 amp current(after a 2 second induction period) ramps up to 7 amps over a period of15 seconds. Film nucleation marginally better than FIG. 47 is observed,but still classified as poor.

FIG. 49 duplicates the plating conditions of FIG. 47 but for a featurehaving a smaller AR.

FIG. 50 duplicates the plating conditions of FIG. 48 but for a featurehaving a smaller AR.

FIG. 51 shows 2 second induction delay in applying current in phase 1,entry phase. Two step application of current is depicted, 1 amp for 15seconds followed by 7 amp.

FIG. 52 shows various induction delays in applying current in phase 1,entry phase. these figures depict that a low current initial stage isimportant although an induction delay prior to application of the first(low) current level is to be avoided.

DETAILED DESCRIPTION OF THE INVENTION

In the following description and figures, similar reference numbers areused to identify similar elements.

The present invention is directed towards the plating of copper onto thesurfaces of high AR features as would typically be encountered in thefabrication of integrated circuits. However, it is not the intent of thepresent inventors that the invention be inherently so limited.Procedures described herein will find application in other areas of thefield of electrodeposition. However, to provide concrete and definiteexpression, we describe in detail herein, the plating of copper.

Typical electroplating baths will contain the metal to be plated, theassociated anions in a typically acid solution. Copper electroplating istypically performed from a solution of CuSO₄ dissolved in an aqueoussolution of sulfuric acid. In addition to these major constituents ofthe plating bath, it is common for plating baths to contain severaladditives. “Additives” is intended to mean herein any type of compoundadded to the plating bath to change the plating behavior. Additives aretypically, but not exclusively, organic compounds. Typically, additivesare present in low concentrations in the plating bath. Additiveconcentrations in the range of parts-per-million (ppm) are typical,perhaps 20 to 400 ppm.

There are three general types of plating bath additives of interest inthe practice of the present invention. Suppressor additives retard theplating reaction, increasing the polarization. Typical suppressors maybe large molecules (for example, polymers) that may have an ionic endgroup, for example a surfactant. Increasing the surface polarizationlayer prevents the copper from adsorbing onto the surface. Suppressorscan thus function as blockers. Suppressors cause the resistance of thesurface to be very high in comparison to the electrolyte diffusion orelectrical resistance. Often ppm levels of chloride ion are required forsuppressers to be effective.

Accelerator additives (catalysts) accelerate the plating reaction.Accelerators may be rather small molecules, perhaps containing sulfurand need not be ionic. Accelerators absorb onto the surface and increasethe current. Accelerators may occur not as the species directly added tothe electroplating bath, but as breakdown products of such molecules. Ineither case, the net effect is to increase current flow and acceleratethe reaction when such species are present (or become present throughelectrochemical breakdown).

Levelers behave like suppressors but tend to be more electrochemicallyactive than suppressors and typically are consumed during plating.Levelers also tend to accelerate plating on depressed regions of thesurface undergoing plating, tending thereby to level the plated surface.The above descriptions of additives are intended to be qualitative only.Specific additives are described elsewhere herein.

In addition, additives adsorbed on the surface may undergo breakdown asvoltage is increased, forcing current through the surface in spite ofthe adsorbed species. These breakdown products typically accumulate inrecessed regions of the surface (that is, in regions less accessible todiffusion of species from the bulk electroplating solution). Suchaccumulation of breakdown products in recessed regions aid in thatregion becoming active for plating in such recessed regions.

The additives used in the practice of the present invention affectplating behavior both by their chemical and physical behavior. Additiveschemically affect the plating behavior according to their intent.However, additives undergo chemical transformation to a greater orlesser extent as the reaction proceeds. The chemical and physicalproperties of the reaction products may also have an effect on theplating processes of the present invention.

The present invention is concerned chiefly with the filling of small,recessed features typically having high ARs. Thus, the present inventionis directly concerned with properties of the plating bath and theplating process inside vias and trenches. Diffusion of species in andout of such regions is an important consideration in the presentinvention as described in detail elsewhere herein.

One of the first considerations in evaluating the effects of additiveson plating is to estimate the amount of additive present in vias andtrenches before plating begins. That is, when the surface to be platedis introduced into the plating bath, what amount of additive materialmay be expected to be found inside vias and trenches? This estimate isgiven in FIG. 15. FIG. 15 gives typical ppm levels for additives inplating baths and the estimated amount of additives to be found in viasand trenches. The calculations of FIG. 15 lead to the conclusion thatthere is insufficient additive initially resident in the interior oftypical features to coat the interior walls of the features by asignificant fraction of a molecular monolayer. In other words, if alladditives initially present inside features were to deposit on the wallsimmediately at time t=0, there would be insufficient concentration onthe walls to have a significant effect of the type the additive isintended to have. Thus, the effect of additives on the plating behaviorinside features is subject to a time delay or induction period. Duringthis induction period, additional additive molecules diffuse into thevias and trenches to deposit on the walls thereof and cause the intendedbehavior. However, it is important in the practice of the presentinvention that the additive induction period inside trenches and vias berecognized and utilized. FIG. 16 estimates that this induction period isof the order of 2 to 3 seconds. These numbers are intended to bequalitative estimates only to demonstrate an induction period of theorder of seconds rather than microseconds or hours. Different inductionperiods will occur for different additive species depending on theirdiffusivity in the plating bath and under the particular conditions ofcurrent and voltage. Ionic additives (for example, chloride) and smallorganic molecules will be expected to diffuse more rapidly than large,uncharged species. But, for an induction period of the order of seconds,the walls of the features to be plated seem to be in contact with asubstantially additive-free electrolyte. Thus, during this inductionperiod the walls of the features are taking up (adsorbing) additionaladditives and increasing the property the additive brings to the platingprocess.

There are other factors tending to keep features additive-free inaddition to the small amount of additive initially brought into thefeature with the plating bath. Typically, additives will be adsorbedonto the walls of the features as well as onto th e (much larger) fieldregion. As more additive diffuses to the surface from the plating bath,adsorption onto the field region will predominate over diffusion intothe feature and adsorption onto the feature walls. In addition, with thepassage of time the additives adsorbed onto field regions will bechemically broken down, thereby freeing adsorption sites on the easilyaccessible field region. The net effect is that the interior of thefeatures plates as if plating from an additive-free bath for someinduction period at the beginning of the plating process beforeadditives reach equilibrium between residence in the plating bath andadsorption on the feature walls. See FIG. 17.

In accordance with the present invention, a method of electroplating ispresented which is particularly well suited for electroplating smallfeatures having high AR. Additionally, embodiments of the presentinvention relate to processes of electroplating a substrate havingfeatures with various ARs in a manner efficiently producing good qualitycoating for all such features.

The present invention typically includes four distinct phases designedto reduce or eliminate one or more problems commonly associated withconventional electroplating of recessed features.

As noted above, the initial PVD copper deposition typically results inthin copper seed layers on the walls and bottoms of trenches and vias.In addition to being thin, the morphology of the coating onwalls/bottoms of trenches/vias can be non-uniform, consisting ofisolated islands of copper lying on an underlying layer of tantalum (orother material). If exposed to oxygen or water, a tantalum oxide layermay form. This oxide layer may constitute the entire thickness of the Tafilm. The inventors have observed that placing a substrate having suchislands into an electrolyte plating bath generally exacerbateswall/bottom non-uniformity, likely due to spontaneous electrochemicaletching. Thus, the first phase, referred to as “the entry phase,”includes procedures to reduce or avoid etching and corrosion of thecopper seed layers within the features and thereby maintain theintegrity of the seed layer for subsequent phases.

As the first, entry, phase avoids exacerbating the wall/bottomnon-uniformity, the second or “initiation phase” tends to selectivelyrepair the non-uniformity in the recessed features. This initiationphase includes nucleation and growth resulting in island-bridging of theseed layer non-uniformity within the features. During the initiationphase, the seed layer within the features is overlaid with another layerthat is more uniform and has desired bottom-up filling properties.

The features next must be filled with conductive material in such amanner as to reduce or eliminate the formation of seams and/or voids. Itis found that features having ARs typically in excess of 0.5 (“high AR”)require different electroplating conditions from those having low ARs(less than approximately 0.5) in order to achieve electroplating withseam/void reduction (or elimination). It is useful to describe thepresent invention in terms of the different conditions required forfilling high and low AR features as two separate phases. However, ashigh AR features fill according to the desired bottom-up manner, thefeature's AR decreases. In actual practice of the present invention insome of its embodiments, the electroplating conditions are changed in acontinuous manner to achieve filling of first high then low AR featuressmoothly. However, to make the discussion more definite, we consider thefilling of high AR features separate from filling of low AR features.

The filling of high AR features according to the present invention isreferred to as “the bottom-up electrofilling phase.” This phase involvesthe deposition of electrically conductive material preferentially ontothe bottoms of the high AR features. The formation of seams and voids isreduced or avoided for features of all sizes and ARs in this phase whilepreferentially electroplating conductive material onto the bottoms ofhigh AR features.

The fourth phase, referred to as “the low AR feature filling phase,”involves the electroplating of electrically conductive material into lowAR features, typically with an AR less than 0.5. This fourth phase iscarried out pursuant with the present invention with thereduction/elimination of seams/voids.

Some embodiments of the present invention can be practiced without theuse of high-current pulsed power supplies. This results in cost savingsboth in the use of less expensive power supplies and also in avoidingthe costs of associated hardware (wiring, contacts, etc.) typicallyrequired by high-current pulsed power supplies.

1. The Entry Phase

FIG. 5A is an enlarged view of the region 100 of FIG. 2 illustratingearly stage degradation of seed layer 20 as observed by the inventors.In this example for purposes of providing a concrete illustration, seedlayer 20 includes a tantalum layer 102 and a copper layer 104.Illustratively, tantalum layer 102 may have a thickness typically in therange of 200-350 Å on field region 22 and copper layer 104 may have atypical thickness in the range of 800-2000 Å on field region 22 althoughlayers of other materials and/or having other thickness can also beused. Thicknesses in trenches and vias are typically 10% of the fieldregion values. As shown in FIG. 5A, copper layer 104 is often seen toform agglomerates or islands 106 of copper. Consequently, regions oftantalum layer 102 are exposed between islands 106.

FIG. 5B is an enlarged view of the region 100 of FIG. 2 illustratingcorrosion of copper layer 104 and oxidation of tantalum layer that islikely to occur in conventional electroplating procedures. As shown inFIG. 5B, the exposed regions of tantalum layer 102 are subject tooxidation, thereby forming patches or regions 108 of tantalum oxide intantalum layer 102. Regions 108 are thus regions of relatively highelectrical resistivity compared to the remaining unoxidized portions oftantalum layer 102. This behavior of tantalum layer 102 represents oneimpediment to adequate and uniform distribution of electroplatingcurrent to bottom 16B of via 16 (FIG. 1).

Referring now to both FIGS. 5A and 5 B, and FIG. 18, copper layer 104 istypically continuous near the top of the feature and tends to bediscontinuous (“island-like”) near the bottom of the feature. As notedabove, shadowing by the feature walls and diffusion-accessibilityexplain preferential copper deposition near the top of features. Theunderlying Ta layer is converted to oxide, 108, where exposed tohumidity, water or oxygen. Thus, a region comprising islands ofconductor, 106, atop a continuous layer of oxide, 108, acts largely asan electrical insulator, failing to distribute electroplating current tobottom of features. The reactions of interest are: (1) 2Ta + 5H₂O →Ta₂O₅ + 10 H⁺ + 10 e⁻ E₀ = −0.75 V (2) 10 H ⁺ + 10 e⁻ → 5 H₂ E₀ = 0.00 V(3) 2Ta + 5H₂O → Ta₂O₅ + 5 H₂ ΔE = 0.75 Vresulting in an insulating layer of Ta₂O₅ , exacerbating the difficultyin getting electricity to the bottoms of the sidewalls of the feature.This is among the problems making plating of the interior walls of viasand trenches, especially the lower walls, particularly difficult. Wehave effectively isolated islands of copper atop an insulating layer oftantalum oxide.

FIG. 19 illustrates an equivalent circuit model for the electricalbehavior of the interior surfaces of features. R_(f) is the filmresistance which is reasonably high due to the presence of tantalumoxide along the sidewall. There are two interfacial resistances, R_(ct)that indicates the resistance of electron transfer across the interface,charge-transfer resistance, or the coupling resistance between a copperion in solution and an electron on the surface combining with the copperion. W denotes diffusion resistance for copper ions. In the presence ofa tantalum oxide surface, R_(f) is large and electrons prefer the routeat the top of the trench, leading to top-of-trench plating,bottlenecking and a poor filling of the feature.

FIG. 20 presents model calculations for the effective electricalresistance and voltage drops to be expected to occur in the filling ofsmall features. Two conclusions result. If metallic tantalum remains,electrical resistance to the bottom of the feature is small. If tantalumoxide is present, the electrical resistance to the bottom of the featurewill be large and require special processing to support bottom-upfilling.

The inventors observe that when substrate 10 is placed in anelectroplating bath, further degradation of the copper layer withintrenches and vias is the typical result. This may be due to a corrosionreaction quickly oxidizing copper layer 104. The corrosion reaction mayinvolve a redox reaction between dissolved oxygen in the electrolyte andcopper layer 104, with electrons flowing from the point at the surfacewhere oxygen is reduced, to islands 106 in via 16 which arepreferentially corroded. The reducing component of this redox reactionis not limited to regions near islands 106 in via 16. In fact, fieldregion 22 is the more likely source of the majority of the reductionreaction because field region 22 is better exposed to replenished oxygenfrom the flowing electrolyte (assuming that seed layer 20 has sufficientelectrical conductivity to carry the electrons from field region 22 toislands 106). The half reactions of interest are: (1) O₂ + 4H⁺ + 4e⁻ →2H₂O E₀ = 1.229 V (vs Standard Hydrogen Electrode, “SHE”) (2) Cu⁰ →Cu⁺² + 2 e⁻ E₀ = 0.342 V (vs SHE) (3) 2Cu + O₂ + 4H⁺ → 2Cu⁺² + 2H₂O E₀ =0.887 V (1 m/l H⁺ and Cu⁺²)

Thus, placing a substrate coated with seed layer 20 into the platingelectrolyte under open circuit conditions results in copper etching(corrosion).

Copper etching is particular pronounced for islands 106 in via 16 (orother feature) which can be understood as a combination of twoeffects: 1) the relatively larger surface energy of curved islandscompared to planar layers; and 2) the larger surface area of coppertypically exposed to the electrolyte (per unit of substrate area) inregions rich in islands in comparison to planar regions. In particular,assuming a hemispherical island 106 (phase 1, copper in this embodiment)on a layer 102 (phase 2, tantalum in this embodiment) surrounded by aliquid (phase 3, electroplating solution in this embodiment), the totalfree energy of formation of the island 106 as a function of the islandradius r, ΔG_(t), isΔG _(t) =πr ²(2σ₁₃ +σ ₁₂−σ₂₃)+(⅔)πr ³ ΔG _(v)   (4)where σ₁₂, σ₁₃, σ₂₃ are the surface energies (tensions) of the variousinterfaces, copper-tantalum, copper-solution, tantalum-solutionrespectively. ΔG_(v) is the volumetric free energy change associatedwith the formation of island 106 with respect to a given solution. Ifequation (4) is expressed on a molar basis, one obtains (in joules permole of material of island 106):[ΔG _(t) /v _(m)]={[3(2σ₁₃+σ₁₂−σ₂₃)]/(2rv _(m))}+ΔG _(v)   (5)where v_(m) is the molar volume of copper.

From equation (5) one can readily derive the thermodynamic potential offormation of an island 106 with a particular radius r with respect to aninfinitely large island (r→∞): $\begin{matrix}\begin{matrix}{{{E\quad(\infty)} - {E\quad(r)}} = {\left\lbrack {({RT})/({nF})} \right\rbrack\quad\ln\quad\left\{ {{- \Delta}\quad{G_{t}/v_{m}}} \right\}}} \\{= {\left\lbrack {({RT})/({nF})} \right\rbrack\quad\ln\quad\left\{ \left\lbrack {\left\lbrack {3\left( {{2\sigma_{13}} + \sigma_{12} - \sigma_{23}} \right)} \right\rbrack/\left( {2{rv}_{m}} \right)} \right\} \right.}}\end{matrix} & (6)\end{matrix}$Equation 6 illustrates that a significant potential difference existsbetween a particle having a small radius r and a planar surface of thesame material. That is, a voltmeter connecting the two would show asignificant voltage reading. Since islands 106 are electricallyconnected to copper layer 104 on field region 22, islands 106 tend todissolve at the expense of forming the larger copper layer 104 on fieldregion 22. Thus, upon being placed into the electroplating solution,copper layer 104 will tend to be etched and, more particularly, islands106 of copper layer 104 will be preferentially etched relative to copperlayer 104 on field region 22. This etching of copper on sidewalls 16S ofvia 16 further increases the electrical resistance leading to the bottomof trenches and vias, providing another impediment to distribution ofelectroplating current to the bottoms of features. This nucleationeffect is depicted in FIG. 21. To prevent this growth of copper islands,a cathodic potential should be applied of sufficient magnitude toovercome the effects of Eq. (6).

Therefore, destruction or degradation of the relatively thin copperlayer on sidewalls 16S of via 16 (and other features) is detrimental toachieving an effective electroplating process. In accordance with thepresent invention, copper layer 104 is preserved by cathodicallypolarizing seed layer 20 relative to the solution. In one embodiment,seed layer 20 is cathodically polarized by pre-setting a power supply toprovide a small cathodic current typically in the range of about 0.1 to5 mA/cm² which flows from the anode to seed layer 20 immediately uponthe seed layer 20 contacting the electrolyte. The actual current valuewill depend on the plating solution, feature density, quality of seedlayer and other conditions. In another embodiment, a slightly cathodicvoltage (i.e. a negative voltage such as −10 mV) with respect to areference electrode (for example, Cu wire in the electrolyte) is appliedto seed layer 20 prior to seed layer 20 making contact with theelectrolyte. The present invention demonstrates that substantialimprovement in feature electroplating is achieved by these procedures.

Through the use of the entry phase described above, copper islands 106and thin copper layer 104 tend to remain intact upon entry of thesubstrate into the electroplating bath. However, even if islands 106 arepreserved, seed layer 20 may not have sufficient electrical conductivityto distribute electroplating current to bottom 16B of via 16 to achievebottom-up filling. We note that this entry phase may not preventoxidation of the tantalum layer since the oxidation may occur due toenvironmental conditions well before the electroplating process begins.

As noted, one embodiment of the process of phase 1 is to cathodicallypolarize the substrate prior to immersion in the plating bath. That is,as soon as the plating bath contacts the substrate, the cathodicpolarization is in place and begins its effect. An alternativeembodiment is to immerse the substrate unpolarized into the plating bathand, after a few seconds, apply cathodic polarization. It may bepreferable in certain instances to permit the substrate to be fullywetted by the plating bath, bubbles dissipated, etc. before cathodicpolarization is applied. The effects of such tests are given in FIGS.38-40 waiting about 2 seconds before current is applied. As shown, adelay of as long as 2 seconds seems to cause loss of copper. Thus, anypause before application of cathodic polarization to the substrate needsto be quite short.

Depicted in FIGS. 47-52 is the effect of applying a cathodicpolarization with a low current value then increasing to a highercurrent value. As shown in the figure, this two step current procedure(applied substantially contemporaneously with, or before, contacting theplating bath), leads to somewhat improved preservation of the copper.Thus, increasing current in time, perhaps in steps or ramping up, seemsto better preserve the copper.

The above system makes use of two electrodes. The cathode, or workingelectrode, is the substrate to be plated. The second electrode, anode,or counter electrode in this case is a copper electrode, providing thesource of plating copper as well as the second electrode. Other types ofcounter electrodes that do not supply copper can be used, such as Ti,Pt, etc. A third or reference electrode may optionally be included thatcarries no current. The reference electrode is typically used as a probefor measurement of voltage, or in controlled-potential processes. Thephase 1 procedures discussed herein needs to be polarized negative withrespect to the reference electrode (solution potential).

2. The Initiation Phase.

The purpose of the initiation phase is to nucleate and to grow arelatively thin conducting film in a conformal manner onto a relativelypoor substrate surface (such as a surface with islands or a spotted filmas depicted in FIG. 5).

FIGS. 37 and 41 depict the effects of low currents under conditions sothe relative surface resistance of the upper and lower portions of thefeature sidewall is high compared to the diffusion resistance. At lowcurrents,.the relative resistance of surface reaction is high, leadingto conformal plating inside the feature. The rate of plating issufficiently low at every position along the sidewall that there is nosignificant depletion effect anywhere. The depletion effect relates tocopper diffusing to the surface at a rate slower than the copperreduction and plating on the surface, resulting in copper diffusionbeing the step limiting the rate of copper plating. However, the currentis intentionally kept sufficiently low herein that the plating rate issmall compared to the diffusion, leading to substantially conformalplating everywhere inside the feature.

In addition, at very low currents, suppression additives are notsignificantly depleted. Low current leading to slow plating (slowcompared to the rate of diffusion of copper and additives to thesurface) results in no preference for copper or suppression additivesfor either the top or the bottom of the feature. Thus, the effectiveresistance on the surface remains large and substantially constanteverywhere, leading to conformal plating. Conformal plating permitsincreased currents to be delivered to the entire feature depth of thefeature. Increasing the current at this stage leads to depletion ofsuppression additives preferentially at the bottom of the feature (wherereplenishment through diffusion is slower than near the top of thefeature). Bottom-up filling is the result, described in more detail asphase 3. Examples of both conformal and bottom-up filling are given inthe figures.

FIG. 6 is an enlarged view of the region 100 of FIG. 2 subsequent to theinitiation phase in accordance with the present invention. As shown inFIG. 6, a conductor layer 110 has been formed on copper layer 104.Conductor layer 110, typically formed of the same electricallyconductive material as layer 104, e.g. copper, has thickness sufficientto support electroplating current distribution to bottom 16B of via 16.Illustratively, conductor layer 110 will typically be relatively thin,having a thickness of up to about 500 Å (Angstroms).

Three procedures have been found by the inventors to perform adequatelyin this initiation phase. All three make use of additives typically usedin electroplating solutions, commonly mercaptopropane sulfonic acid,dimercaptopropane sulfonic acid and the like that aid in the nucleationprocess (typically referred to as brighteners, nucleation catalysts oraccelerators), and suppressors (such as polyethylene glycol,polyethylene oxide, polypropylene oxide, mixtures and co-polymersthereof).

In the first embodiment, conductor film 110 is formed by applying a lowdc net cathodic current, illustratively in the range of 0.1 to 5 mA/cm².Using a relatively low dc net cathodic current allows additives in theelectrolyte to adsorb onto the copper layer 104 from the bulk of theelectrolyte solution. The additives are found not to be significantlydepleted in the bulk solution by this process. Electroreduction causesthe additives to be consumed at relatively low rates due to therelatively low current density applied, sufficiently low that additiveor suppression additives for either the top or the bottom of thefeature. Thus, the effective resistance on the surface remains large andsubstantially constant everywhere, leading to conformal plating.Conformal plating permits increased currents to be delivered to theentire feature depth of the feature. Increasing the current at thisstage leads to depletion of suppression additives preferentially at thebottom of the feature (where replenishment through diffusion is slowerthan near the top of the feature). Bottom-up filling is the result,described in more detail as phase 3. Examples of both conformal andbottom-up filling are given in the figures.

FIG. 6 is an enlarged view of the region 100 of FIG. 2 subsequent to theinitiation phase in accordance with the present invention. As shown inFIG. 6, a conductor layer 110 has been formed on copper layer 104.Conductor layer 110, typically formed of the same electricallyconductive material as layer 104, e.g. copper, has thickness sufficientto support electroplating current distribution to bottom 16B of via 16.Illustratively, conductor layer 110 will typically be relatively thin,having a thickness of up to about 500 Å (Angstroms).

Three procedures have been found by the inventors to perform adequatelyin this initiation phase. All three make use of additives typically usedin electroplating solutions, commonly mercaptopropane sulfonic acid,dimercaptopropane sulfonic acid and the like that aid in the nucleationprocess (typically referred to as brighteners, nucleation catalysts oraccelerators), and suppressors (such as polyethylene glycol,polyethylene oxide, polypropylene oxide, mixtures and co-polymersthereof).

In the first embodiment, conductor film 110 is formed by applying a lowdc net cathodic current, illustratively in the range of 0.1 to 5 mA/cm².Using a relatively low dc net cathodic current allows additives in theelectrolyte to adsorb onto the copper layer 104 from the bulk of theelectrolyte solution. The additives are found not to be significantlydepleted in the bulk solution by this process. Electroreduction causesthe additives to be consumed at relatively low rates due to therelatively low current density applied, sufficiently low that additivereplenishment: by diffusion occurs. The additives adsorbed to copperlayer 104 inhibit electroplating to those regions adsorbing additives,and thus tend to uniformly redistribute the electroplate material ofconductor layer 110. The relatively low dc current is not sufficient tocause bottom-up filling (required in later phases) because, as describedbelow, bottom-up filling requires additive depletion.

Without additives, conductor layer 110 would be preferentially depositedin certain regions, e.g. regions to which electroplating current ispreferentially distributed. Certain additives tend to impedeelectroplating by adding a significant electrical resistance in serieswith other electrical resistance in the electrodeposition circuit. Ifthis additive-created electrical resistance is sufficiently large, thedeposition rate in regions of the surface having additives will bereduced in comparison with those regions lacking additives. Thus, thedeposition rate can be equalized by a proper choice of additive leadingto uniform (conformal) deposition rates. Conducting layer 110 can thushave a relatively uniform thickness. The relative concentration ofnucleation additives (brighteners) can be adjusted with respect to otheradditives to improve the uniformity of conductor layer 110, but the useof low current is found to be the most effective method of achieving thedesired uniformity (that is, currents sufficiently low to avoid additivedepletion).

The second method for performing this initiation phase also involves theuse of relatively small net dc cathodic currents (typically 0.1 to 5mA/cm²) onto which are superimposed short cathodic current pulses in therange of 25 to 250 mA/cm² with pulse lengths in the range of 0.5 to 10ms at a relatively low duty cycle in the range of 0.2 to 10%. Thesesuperimposed pulses aid in the formation between islands that aresubsequently grown to fill the gaps during the low-current (pulse off)portion of the duty cycle.

A third approach involves use of small, net dc cathodic current ontowhich is superimposed an alternating cathodic and anodic current pulseor wave at a moderate duty cycle. For example, an applied low dc netcathodic current in the range of 0.1 to 5 mA/cm² onto which aresuperimposed alternating cathodic and anodic current pulses in the rangeof 25 to 250 mA/cm² with pulse lengths in the range of 0.5 to 10 ms at amoderate duty cycle in the range of 10 to 50%. It is believed thatsuperimposing an alternating cathodic and anodic current pulse mayenhance formation of conductor film 110 better than superimposing only acathodic current pulse in some instances, perhaps by enhancing filmactivation.

Conductor film 110 is also formed on copper layer 104 on field region22. Accordingly, conductor film 110 reduces the electrical resistancefrom the edge of substrate 10 to the various features on substrate 10.(The electrical interconnection between substrate 10 and theelectroplating apparatus is typically established at the edge ofsubstrate 10.)

3. The Bottom-Up Filling Phase.

Highest AR features are the most constricted and fill first in timebecause these features most quickly in time become depleted ofadditives. Additive depletion increases the plating rate, leading topreferential filling of higher AR features in which diffusion ofsuppression additives to the bottom of the feature is slowest. For agiven fixed current density, higher AR features lead to depletion mostquickly and plating starts to accelerate earlier in time. Very smallaspect ratio features require a higher current to lead to bottom-upfilling, because they are less restrictive to diffusion of suppressionadditives to the plating site. Very low AR features plate conformallythroughout. Preferably, the current for bottom-up filling should beginlow to plate preferentially the bottoms of the largest AR features.Ramping up the current leads to plating of smaller AR features in thebottom-up mode as higher rates of additive diffusion are required to“keep up” with the higher plating brought about by increasing current.

Substrates to be electroplated according to the present invention willgenerally have both high and low AR features. The primary focus of thepresent invention is directed towards overcoming the particular problemsoccurring in electroplating high AR features. Nevertheless, in platinghigh AR features, the present invention typically does so in a manner soas to permit subsequent electroplating of low AR features in anefficient manner without seams or voids. Additionally, the presentinvention as practiced on high AR features typically involves fillingsuch features from the bottom-up. Thus, as the depth of the featuredecreases faster than the width, the AR necessarily also decreasesreducing high AR features to low AR features as electroplating proceeds.This bottom-up filling phase is thus directed at preferentiallydepositing conductor on the bottoms of high AR features. The fourth andfinal electroplating phase completes the seamless/voidlesselectroplating of low AR features which, at the beginning of this fourthphase, includes all features, including formerly high AR features nowhaving low ARs.

This bottom-up filling phase makes use of plating additive mixtures toachieve a high degree of “redistribution power” (that is, preferentiallydirecting electroplating to small regions and recesses generallyinaccessible to the plating electric field and to bulk solutionmaterials). It is believed that some of these additives are consumed(reduced) as a necessary side reaction to the main copper depositionprocess. There is also evidence for the interaction between such organicadditive species. Whatever the precise mechanism of interaction, thepresent invention makes use of depletion and diffusion-limitedreplenishment to deposit copper selectively on the bottoms of high ARfeatures.

The bottom-up filling phase essentially makes use of preferentialdeposition of copper on surfaces that may be lacking suppressiveadditives (or concentrating additive breakdown products that may act ascatalysts) in the immediate region. Thus, diffusion through theelectrolyte determines the rate at which additives arrive at the surface(or breakdown products can be removed) while the current flow determineshow rapidly the additives are consumed. For bottom-up filling of high ARfeatures, the phase commences with reasonably low currents. Thus, thebottoms of the most diffusion-constrained features (highest ARs) will bethe only regions substantially lacking such additives and, thus, theonly regions significantly receiving deposited copper. As such regionsfill, less diffusion-constrained regions require electroplating. This isachieved in the present invention by increasing the current flow suchthat additive depletion occurs in the most diffusion-limited regions(still bottoms of features) although less diffusion-constrained than thebottoms of higher AR features (now partially filled). Thus, theessential operation of the bottom-up filling phase involves thefollowing steps: Selecting additives, electrolyte conditions and currentflows such that only the most diffusion-limited regions at the bottomsof the highest AR features receive substantial electroplating instepwise or continuous manner. Increasing the current flow continuously,thereby electroplating progressively less diffusion-limited regions offeatures. Finally, only low AR features remain for electroplating,leading to the fourth and final phase. Specific embodiments follow.

FIG. 7 is a cross-sectional view of substrate 10 along the line II-II ofFIG. 1 after the initiation phase in accordance with the presentinvention. As shown in FIG. 7, conductor layer 110 now covers fieldregion 22 and lines trench 14, vias 16, 18. (Although only conductorlayer 110 is labeled and illustrated in the subsequent figures forpurposes of clarity, it is understood that seed layer 20 underliesconductor layer 110.) As set forth above, conductor layer 110 has athickness and electrical conductivity sufficient to distributeelectroplating current to bottoms 14B, 16B, 18B of trench 14, vias 16,18, respectively.

At this point, if a relatively high electroplating current were appliedto conductor layer 110, voids and seams in trench 14, via 16, 18 similarto conventional seams 28, 30 and void 26 as illustrated in FIG. 4 arelikely to result. That is, this is thought to be due to the fact thatadditives contained in the electroplating solution within the featureswould deplete over much of the feature wall and bottom surface in thepresence of high currents, and, due to the constrained flow in thefeatures, not be promptly replenished. This depletion would negate theleveling effect of the additives on the deposition of metal resulting indeposition of the electroplated material at the same rate on the featurebottom as the feature sidewall. Since the features are much deeper thanwide, seams or voids would tend to be formed as the walls constrict thelateral extent of the feature more rapidly than it fills. Further,diffusion limitations of the ions of material to be electroplated causethe electroplated material to be preferentially deposited in thefeatures nearer to the field region 22 rather than closer to the bottomof the feature, thus enhancing formation of seams and voids. Avoidanceof seams and voids is a primary feature of the present invention,negated in this phase if relatively high currents are used.

To avoid seam and void formation, the features are preferentially filledfrom the bottom-up. Referring again to FIG. 7, to effect bottom-upfilling, substrate 10 is immersed in an electroplating solution 112 ormore typically remains in solution 112 from the previous initiationphase. Solution 112 contains electroplating additives 114 such asmercaptopropane sulfonic acid(MPS), dimercaptopropane sulfonic acid(SPS), N-N dimethyl dithiocarbamic acid-3-sulfopropyl-ester (DPS),polyethylene glycol and/or polypropylene glycol. Commercial productssuch as Enthonem (OMI, Inc., New Haven, Conn.), Cu-Bath-SC™ andCu-Bath-N™, Shipley Ronal (Marlborough Mass.), Copper Ultrafill™, tomention just a few, may also be usefully employed in the practice of thepresent invention. Only a single additive 114 is labeled for purposes ofclarity. However, additives can be used singly or in combination as amixture of distinct chemical species. At this point in the process ofthe present invention, additives 114 are essentially uniformlydistributed in solution 112 and in trench 14, vias 16, 18.

FIG. 8 depicts the cross-sectional view of FIG. 7 during the initialstage of a bottom-up filling phase in accordance with the presentinvention. Referring now to FIG. 8, there is initially a time foradsorption of additives to occur before an initial electroplatingcurrent is applied to conductor layer 110. Following this finitetransition after the part is placed into the plating solution,electroplating current is applied. For example, a dc current in therange of approximately 1 to 300 mA/cm² may be applied to conductor layer110.

Of importance, at essentially all expected concentrations of additive114, e.g. between 20 to 300 ppm contained in the initial solution, thereis insufficient additive 114 stored in via 16 to lead to substantialsurface absorption of additive 114 on conductor layer 110 in via 16. By“substantial surface absorption” we intend the formation of a monolayerof additive 114 on conductor layer 110. Thus, additive 114 must diffusefrom the bulk of solution 112 and into via 16 to allow substantialsurface absorption of additive 114 on conductor layer 110 in via 16since a boundary layer typically separates the substrate from the bulkof the solution and the concentration of various species in the bulk ofthe solution is substantially unaffected during the electroplatingprocess. The diffusion of additives 114 from the bulk of solution 112into via 16 takes a significant amount of time, e.g. on the order ofseconds.

FIG. 9 is the cross-sectional view of FIG. 8 further along in thebottom-up filling phase in accordance with the present invention. Asshown in FIG. 9, a significant amount of additive 114 has adsorbed onsidewalls 18S and bottom 18B of via 18. As alluded to above in theinitiation phase discussion, adsorbed additives tend to inhibitelectroplating. In the case when copper is to be electroplated, thesignificant amount of additive 114 adsorbed on conductor layer 110 invia 18 prevents any appreciable copper deposition in via 18.

In the absence of current, the additive materials will eventually beadsorbed essentially everywhere. One approximate model is that theadditive is consumed by incorporation into the surface or conversioninto another compound at the surface. The applied current essentiallyestablishes a unique potential in the metal phase (everywhere along thesurface and in the trench). The rate of local consumption of additive isa function both of electrical potential and accessibility of species tothe surface by diffusion (diffusion accessibility). In recessed regions,diffusion is relatively poor causing the additive to be replaced at aslower rate than it is consumed. Thus, in such additive-poor regionsthere is little current suppressing effect and a resulting high current.

According to one model, it is believed that additives 114 are consumed,e.g. reduced, during electroplating. In particular, it is believed thatadditives 114 are consumed as a necessary side reaction to theelectroplated copper deposition reaction. The additive 114 consumed mustbe replenished by diffusion of additive 114 from the bulk of solution112. As shown in FIG. 8, additive 114 diffuses into via 16 andreplenishes any additive 114 consumed in via 16 near field region 22.However, there is insufficient diffusion of additive 114 into bottom 16Bof via 16 to replenish additive 114 consumed on and near bottom 16B.Diffusion of additive 114 to or near bottom 16B of via 16 is limited dueto the relatively high AR of via 16. This phenomenon can be contrastedto that observed in via 18 which has a low enough AR to allow anyadditive 114 consumed in via 18 to be readily replenished from the bulkof solution 112.

Since additive 114 is substantially depleted near bottom 16B of via 16,copper is deposited on bottom 16B and sidewalls 16S adjacent bottom 16B.Thus, copper begins to fill via 16 from bottom 16B. Further, since anyadditive 114 consumed in via 16 near field region 22 is readilyreplenished from the bulk of solution 112, significant deposition ofcopper on sidewalls 16S near field region 22 does not occur. The netresult is that copper fills via 16 from bottom 16B and up. This is theso called “bottom-up filling” in accordance with the present invention.

The surface concentration profile of additive 114 in any feature dependson several factors such as the additive absorption time, flowconditions, local current distribution, film resistance, etc. However,to a large extent, the surface concentration profile depends primarilyon the AR of the feature. To illustrate, assuming that trench 14 has thesame AR as via 16, trench 14 also shall experience bottom-up filling butpossibly to a lesser extent since additive 114 is depleted on the entirecircumference 16S of via 16 but only on two sidewalls 14S of trench 14,a ratio of 4/2 for square vias or π/2 for cylindrical vias, favoringdepletion in vias in both cases. Thus, as shown in FIG. 9, the optimalset of conditions for bottom-up filling often varies depending upon theparticular feature. Further, over time, as via 16 is filled from bottom16B up, the effective AR of via 16 decreases. Eventually, assuming a netcurrent, additive depletion in via 16 would be readily replenished thusdecreasing or eliminating the bottom-up filling selectivity.

FIG. 10 is the cross-sectional view of FIG. 9 further along in thebottom-up filling phase in accordance with the present invention. As setforth above, if the bottom-up filling electroplating current wasmaintained at its initial value, bottom-up filling selectivity wouldbegin to diminish for low AR features such as the now partially filledvia 16 and trench 14. To maintain bottom-up filling selectivity for lowAR features (or to obtain bottom-up filling selectivity for low ARfeatures previously lacking such selectivity such as via 18), theelectroplating current is increased. Illustratively, the bottom-upfilling phase is initiated with a dc current in the approximate range of0.0 to 5.0 mA/cm² and increased over a period in the range of 3 to 60seconds to a maximum dc current in the approximate range of 4 to 45mA/cm².

Referring to FIG. 10, as the electroplating current is increased, theelectroplated copper deposition rate increases as does the rate ofconsumption of additive 114. Consequently, diffusion of additive intovia 18, which was previously sufficient to readily replenish anyadditive 114 consumed in via 18, now becomes insufficient to replenishdepletion of additive 114 near bottom 18B of via 18. Since additive 114is substantially depleted near bottom 18B, copper is deposited on bottom18B and sidewalls 18S adjacent bottom 16B. Thus, copper now begins tofill via 18 from bottom 18B. Further, since any additive 114 consumed invia 18 near field region 22 is readily replenished from the bulk ofsolution 112, significant deposition of copper on sidewalls 18S nearfield region 22 does not occur. The net result is that copper fills via18 from bottom 18B and up. Trench 14, via 16, which now have lower ARs,also continue to fill from bottoms 18B, 16B, respectively, for similarreasons. Thus, by increasing the bottom-up filling electroplatingcurrent over time, optimal bottom-up filling results regardless of thecharacteristics (e.g. AR) of the feature.

In certain cases where features are particularly difficult to fill suchas features having extremely high ARs or bottle-necks, the applicationof anodic pulses during the bottom-up filling phase avoids seam and voidformation. FIG. 11 is a graph of current versus time for an illustrativebipolar pulse plating waveform in accordance with the present invention.FIG. 12 is a cross-sectional view of via 16 of FIG. 10 illustrating theredistribution of electroplated layer 116 using the bipolar pulseplating waveform depicted in FIG. 11.

Referring to both FIGS. 11 and 12, initially a cathodic current I_(c) isapplied to conductor layer 110 for a time period t_(c). Illustratively,cathodic current I_(c) can be in the range of 3 to 45 mA/cm² and timeperiod t_(c) can be in the range of 1 to 200 msec. In one particularembodiment, cathodic current I_(c) is 15 mA/cm² and time period t_(c) is100 msec. Multiple cathodic pulses with off times and multiple anodicpulses with off times can also be used, depending on the desired grainstructure, growth rates and bottom-to-top selectivity desired. Duringthe application of the cathodic current I_(c), via 16 is filledselectively from bottom 16B as discussed above.

Following application of cathodic current I_(c), no current is appliedduring an off-time period t_(offc-a) during which relaxation andreplenishment of additive 114 and copper ions in via 16 occurs(additives 114 are not shown in FIG. 12 for purposes of clarity).

An anodic current I_(a) is then applied to conductor layer 110 for atime period t_(a). It is believed that time period t_(a) should besufficient to establish diffusion-controlled conditions of additivewithin the feature during the anodic pulse and that, on the subsequentcathodic pulse, sufficient time is allowed for the re-establishment ofdiffusion limited transfer of additives to the bottom of the feature.The anodic current pulse as depicted in FIG. 11 preferentially removescopper from the upper portion of feature walls. Thus, a plating waveformof the type depicted in FIG. 11 includes a portion (the anodic portion)tending to compensate for preferential plating on the upper portions ofthe sidewalls by preferentially de-plating metal for the same region.Numerous waveforms similar to that depicted in FIG. 11 may be employed.The reverse pulse waveform needs to be high enough so that substantialdiffusion, back-resistance inside the trench occurs. The length of thepulse is determined by longer than about 10 msec. The relation tocharges carried in the forward and reverse pulses needs to be greaterthan 1 to result in net plating (not de-plating). Thus, the negativearea under the curve in FIG. 11 needs to exceed the positive area. FIGS.22-36, 41-46 are examples of filling with various anodic pulses underconditions depicted on the figures and described in the description offigures.

4. Low AR Filling Phase.

In addition to previously high AR features (now partially filled) verysmall AR features may occur on integrated circuits (“pads”). Forexample, a feature requiring metal filling may be 1:50 depth to width.Such gaping features will not have been significantly affected by thepreceding electroplating steps. However, it is the function of thisfourth phase to fill such features as well as finish the filling ofthose features whose filling has begun in previous phases. In these verysmall AR features, the metal must be plated to a level that the metalrises to a level higher than the field oxide layer. In essence, thethickness of the metal electrodeposited must exceed the depth of the padsuch that, upon subsequent planarization, the metal remains above thepad to the desired thickness. In terms of processing speed, this padfilling process is the most time consuming. For rough qualitativeestimates, phase 1 is of the order of a few seconds. Phase 2 is of theorder of 10-20 seconds. Phase 3 is of the order of 30 seconds. And thepad (or “bulk”) fill, phase 4, is of the order of 90 seconds. Thus,since overall process time is a consideration, performing the bulk fillas rapidly as possible is advantageous.

Once essentially all of the vias and trenches having high ARs have beenfilled, features with low AR (typically less than about 0.5) need to befilled. This generally is done by substantially conformal filling sincethe electroplating process is typically governed by electric field anddiffusion dependent mechanisms during which additive depletion or sidewall closure is not likely. Therefore, a layer of metal approximatelyequal to the dielectric layer in thickness is generally deposited(typically between 0.7 and 1.4 mm). Use of high currents increases thethroughput of the process so long as the currents are not so high as tolead to significant reduction in anode service life or to additivemaintenance/degradation. Also, compensation for non-uniformity arisingfrom field shaping, wafer holder design, shielding, etc. is typicallydone at this phase of the electroplating process. Typical plating ratesrange from 15 to 75 mA/cm², more typically from 20 to 50 mA/cm², andmost typically 25-40 mA/cm². Metal deposited during this phase of theprocess is commonly removed in part in a subsequent metal planarizationstep.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific and preferred embodimentsillustrated and described. Rather, it is intended that the scope of theinvention be determined by the appended claims.

1. A method of electroplating a metal onto a surface comprising a fieldregion and a recessed feature, the method comprising: a) contacting saidsurface with an electroplating solution comprising metal ions and anadditive; and b) applying a cathodic current through said surface, thecathodic current being sufficiently small that depletion of metal ionsand the additive is absent at both the field region and the recessedfeature, resulting in substantially conformal electroplating of saidsurface.
 2. A method of electroplating as in claim 1 wherein saidcathodic current is a dc current in the range from approximately 0.1milliamperes per square centimeter to approximately 5 milliamperes persquare centimeter.
 3. A method of electroplating as in claim 2 furthercomprising cathodic current pulses superimposed on said dc cathodiccurrent.
 4. A method of electroplating as in claim 3 wherein saidcathodic current pulses carry currents in the range from approximately25 milliamperes per square centimeter to approximately 250 milliamperesper square centimeter.
 5. A method of electroplating as in claim 3wherein said cathodic current pulses have pulse lengths in the rangefrom approximately 0.5 milliseconds to approximately 10 milliseconds. 6.A method of electroplating as in claim 3 wherein said cathodic currentpulses have a duty cycle in the range from approximately 0.2% toapproximately 10%.
 7. A method of electroplating as in claim 2 furthercomprising alternating cathodic and anodic current pulses superimposedon said dc cathodic current.
 8. A method of electroplating as in claim 7wherein said current pulses carry currents in the range fromapproximately 25 milliamperes per square centimeter to approximately 250milliamperes per square centimeter.
 9. A method of electroplating as inclaim 7 wherein said current pulses have pulse lengths in the range fromapproximately 0.5 milliseconds to approximately 10 milliseconds.
 10. Amethod of electroplating as in claim 7 wherein said cathodic currentpulses have a duty cycle in the range from approximately 10% toapproximately 50%.
 11. A method of electroplating as in claim 1 whereinsaid additive is selected from the group consisting of mercaptopropanesulfonic acid, dimercaptopropane sulfonic acid, N-N dimethyldithiocarbamic acid-3-sulfopropyl-ester, polyethylene glycol,polypropylene glycol, polyethylene oxide, polypropylene oxide, andcopolymers and mixtures thereof.
 12. A method of electroplating a metalonto a surface comprising a plurality of recessed features, the methodcomprising: a) contacting said surface with an electroplating solutioncomprising metal ions and an additive for a time sufficient foradsorption of said additive onto said surface; b) applying a dc cathodiccurrent having an initial value through said surface, the initial valuesuch that electroplating occurs preferentially on bottoms of recessedfeatures having the least diffusion-accessibility; and c) increasingsaid current from said initial value such that electroplating progressesto bottoms of features having higher diffusion-accessibility.
 13. Themethod of claim 12 wherein said additive comprises at least one chemicalspecies that suppresses electroplating when adsorbed on said surface andwherein said additive is transformed upon passage of the dc currentthrough said surface so as to lose said electroplating suppressingactivity.
 14. The method of claim 13 wherein said initial value is suchthat adsorbed additive transformed by said current is replaced bydiffusion of additive to a site of said transformation everywhere excepton said bottoms of recessed features having the leastdiffusion-accessibility.
 15. The method of claim 12 wherein the initialvalue is between about 0 and about 5 milliamperes per square centimeterand increasing said current in step (c) is increasing said current overa period of between about 3 and about 60 seconds to a maximum dc currentof between about 4 and about 45 milliamperes per square centimeter. 16.A method of electroplating as in claim 12 further comprising immediatelyfollowing step b): terminating said cathodic current flow; applying ananodic current pulse; terminating said anodic current; and resumingcathodic current flow.
 17. The method of claim 16 wherein the initialvalue is between about 3 and about 45 milliamperes per square centimeterand applying a dc current is applying a dc current for a time periodbetween about 1 and about 200 milliseconds.
 18. A method ofelectroplating as in claim 12 wherein said additive is selected from thegroup consisting of mercaptopropane sulfonic acid, dimercaptopropanesulfonic acid, N-N dimethyl dithiocarbamic acid-3-sulfopropyl-ester,polyethylene glycol, polypropylene glycol, polyethylene oxide,polypropylene oxide, and copolymers and mixtures thereof.
 19. A methodof mitigating corrosion of a metal layer on the surface of recessedfeatures caused by contact of said metal layer with an electroplatingsolution, said method comprising cathodically polarizing said metallayer with respect to said solution.
 20. A method as in claim 19 whereinsaid cathodic polarization is applied prior to or less thanapproximately 5 seconds following contact of said metal layer with saidelectroplating solution.
 21. A method of mitigating corrosion as inclaim 19 wherein said cathodic polarization of said metal layer isperformed by causing a cathodic current to flow between said metal layerand a counter electrode.
 22. A method of mitigating corrosion as inclaim 21 wherein said cathodic current is in the range fromapproximately 0.1 milliamperes per square centimeter to approximately 5milliamperes per square centimeter.
 23. A method of mitigating corrosionas in claim 20 wherein said cathodic polarization of said metal layer isperformed by applying a net cathodic voltage to said metal layer withrespect to a copper reference electrode in the electroplating solutionprior to said metal layer contacting said electroplating solution.
 24. Amethod of mitigating corrosion as in claim 23 wherein said voltage isapproximately −10 millivolts with respect to said reference copperelectrode in said electroplating solution.
 25. A method ofelectroplating a metal onto a surface comprising a field region and aplurality of recessed features, the method comprising: a) contactingsaid substrate with an electroplating solution comprising metal ions andan additive; b) applying a cathodic current through said surface, thecathodic current being sufficiently small that depletion of metal ionsand the additive is absent at both the field region and the recessedfeatures, to create a substantially conformal conductive film on saidsurface; c) contacting said surface with said electroplating solutionfor a time sufficient for adsorption of said additive onto said surface;d) applying a dc cathodic current having an initial value through saidsurface, the initial value such that electroplating occurspreferentially on bottoms of recessed features having the leastdiffusion-accessibility; and e) increasing said current from saidinitial value such that electroplating progresses to bottoms of featureshaving higher diffusion-accessibility.
 26. The method of claim 25further comprising continuing step (e) until the aspect ratios of all ofsaid recessed features are less than approximately 0.5.
 27. A method ofelectroplating a metal onto a surface comprising a field region and aplurality of recessed features, the method comprising: a) depositing aconducting seed layer on said substrate; b) cathodically polarizing saidseed layer with respect to an electroplating solution comprising metalions and an additive prior to or less than approximately 5 secondsfollowing contact of said seed layer with said electroplating solution;c) contacting said substrate with said electroplating solution; d)applying a cathodic current through said surface, the cathodic currentbeing sufficiently small that depletion of metal ions and the additiveis absent at both the field region and the recessed features, to createa substantially conformal conductive film on said surface; e) contactingsaid surface with said electroplating solution for a time sufficient foradsorption of said additive onto said surface; f) applying a dc cathodiccurrent having an initial value through said surface, the initial valuesuch that electroplating occurs preferentially on bottoms of recessedfeatures having the least diffusion-accessibility; g) increasing saidcurrent from said initial value such that electroplating progresses tobottoms of features having higher diffusion-accessibility. h) continuingstep (g) until the aspect ratios of all of said recessed features areless than approximately 0.5; and, i) conformally plating said surface,filling said recessed features.
 28. A method of electroplating as inclaim 27 wherein said conductive film on said surface achieves athickness of at least approximately 500 Angstroms.